Comparing Sound on Three Flowmaster Muffler Kits on a S197

Starting life 27 years ago – coincidentally the same as the author – Flowmaster has been in the high-performance exhaust game for quite a while. What started off as a small operation has since become one of the most recognizable names in the industry, with an R&D facility that puts some laboratories to shame, almost a half-million square feet of floorspace, and over 250 employees. So it’s no surprise that in the touch over five years since the debut of the 2005 Mustang and the S197 platform, Flowmaster has five part numbers for the 2005-2010 GT fitment. Recently, Flowmaster’s Cam Benty cruised down to powerTV with a trunk full of parts, to do a comparison video of three different systems on an S197 – which is directly below.

Words by Greg Acosta – Race Pages Magazine

For this particular test, there were three part numbers from Flowmaster to be tested:
• The Hushpower axle-back system, utilizing the Hushpower “Pro Series” round mufflers
• American Thunder axle-back kit, utilizing the 40-series classic dual-chamber mufflers
• American Thunder cat-back kit, using the 60-series Delta Flow mufflers

Our S197 Mustang Test Mule

Hushpower Pro Series Axle-Back – PN# 819108

Hushpower, a division of Flowmaster, offers an axle-back system that utilizes a pair of round Pro Series mufflers, with 2.5-inch inlets and 3-inch outlets, part number 819108. The tips are 4-inch polished stainless units that look very nice poking out of the rear bumper cover. The entire kit is made of 409S stainless steel and offers what the company calls a “moderate” sound. At idle it features a mild rumble with a little gurgling and popping when you rev it in neutral. Under load it as a very smooth, mellow tone.

• 409S stainless steel
• Moderate sound, mild resonance
• Oval-style mufflers
• Direct bolt on axle-back

Hushpower axle-back is made from 409S and has a moderate tone

American Thunder 40-Series Classic Axle-Back – PN# 817460

The American Thunder axle-back, utilizing the 40-Series Classic mufflers is the most modestly-priced kit of the group tested. The system is mandrel-bent and the 2.5-inch inlet and outlet 40-Series Classic mufflers have been selected for both their performance and their aesthetics. Again, made of 409S stainless steel, the 40-Series Classic axle-back is true to its old-school roots, in that it is loud – what Flowmaster calls “aggressive.” The 40-Series dual-chamber muffler produces what’s best described and the stereotypical “Muscle Car” sound. At idle it growls, and that growl increases smoothly – albeit loudly – as you free-rev the car. Under load, the 40-Series smoothed out but still has a moderate amount of resonance in the cockpit. This system is the clear choice for someone that wants to be heard.

• 409S stainless steel
• Aggressive sound, Moderate resonance
• Classic square-style mufflers
• Direct bolt on axle-back

You can see the visual difference between the 40-Series Classic two-chamber – which is a remake of the original “suitcase muffler” that put Ray Flugger and Flowmaster on the map back in the ‘80s, and the modern 60-Series Delta Flow muffler.

The American Thunder 60-Series Delta Flow Cat-Back – PN# 817494

The last system we tested was the full American Thunder cat-back exhaust system. Utilizing the 60-Series Delta Flow muffler, the kit uses entirely mandrel-bent, 3-inch tubing from the catalytic converters back. The 60-Series Delta flow mufflers are specially engineered for performance, economy and sound. By themselves, they have a tone Flowmaster describes as “moderate,” but with the addition of the 3-inch piping in the cat-back kit, it gets bumped to “aggressive.” While the volume is a bit louder with the three inch piping, the 60-series muffler offers a smooth, mellow tone with minimal interior resonance, and a deep, dignified rumble outside of the car under load.

• 409S stainless steel
• Aggresive Sound, lower  resonance than the American thunder
• Rounded square-style Delta 60 mufflers
• Maximum power, though requires some cutting

Here you can see all the goodies we got to play with. Two axle-back kits and a full cat-back kit only represent 60-percent of the part numbers Flowmaster has for the 4.6L S197 GT.

Once we installed and removed both of the axle-back kits it was time to cut off the stock exhaust right behind the stock cats, and mount the new cat-back tubing. The joint between stock and new piping is made with wide, band-type exhaust clamps.

While the stock exhaust features an H-pipe to equalize exhaust between the two banks, the Flowmaster kit uses the crossover that is kind of a hybrid between an H-pipe and X-pipe, as you can see here

The fully-mandrel-bent piping fit under the car like a glove. The new piping came in three sectons, making it much easier to handle under the car, rather than trying to manipulate long, ungainly single sections of tubing.

once all bolted up, the full American Thunder cat-back kit looks almost factory. It is a well-engineered, good-fitting kit that is easy to install and sounds great, too. We didn’t get a chance to put the car on the dyno, but if our butt-dyno is calibrated, it definitely added some oomph to our Pony’s giddyup.

Our Thoughts on the Three Kits

After hearing all three, it’s clear that each part caters to a specific person driving an S197 Mustang. The Hushpower axle-back is ideal for someone on a budget who wants a milder sound with a corresponding bump in performance. The 40-Series American Thunder axle-back is ideal for someone on a budget who wants to be heard, and wants his or her new Pony to have the classic muscle car sound. Finally the American Thunder full cat-back with the 60-series Delta Flow muffler offers a dignified performance sound with maximum performance gains. No matter what your tastes, Flowmaster makes a system that caters to both your budget and level of performance desired.

About the author

Mark Gearhart

In 1995 Mark started photographing drag races at his once local track, Bradenton Motorsports Park. He became hooked and shot virtually every series at the track until 2007 until he moved to California and began working as a writer for Power Automedia. He was the founding editor for its first online magazines, and transitioned into the role of editorial director role in 2014. Retiring from the company in 2016, Mark continues to expand his career as a car builder, automotive enthusiast, and freelance journalist to provide featured content and technical expertise.
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